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In physical sciences, standard conditions for temperature and pressure (informally abbreviated as STP) are standard sets of conditions for experimental measurements, to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST) but are far from being universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions. The current version of IUPAC's standard is a temperature of 0 °C (273.15 K, 32 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm), while NIST's version is a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 101.325 kPa (14.696 psi, 1 atm).

In industry and commerce, standard conditions for temperature and pressure are often necessary to define the standard reference conditions to express the volumes of gases and liquids and related quantities such as the rate of volumetric flow (the volumes of gases and liquids vary significantly with temperature and pressure). However many technical publications (books, journals, advertisements for equipment and machinery) simply state "standard conditions" without specifying them, often leading to confusion and errors.

Definitions

Past use

In the last five to six decades, professionals and scientists using the metric system of units defined the standard reference conditions of temperature and pressure for expressing gas volumes as being and 101.325 kPa (1 atm or 760 Torr). During those same years, the most commonly used standard reference conditions for people using the imperial or U.S. customary systems was and 14.696 psi (1 atm) because it was almost universally used by the oil and gas industries worldwide. However, the above two definitions are no longer the most commonly used in either system of units.

Current use

Many different definitions of standard reference conditions are currently being used by organizations all over the world. The table below lists a few of them, but there are more. Some of these organizations used other standards in the past, such as IUPAC which currently defines standard reference conditions as being 0 °C and 100 kPa (1 bar) of pressure rather since 1982, in contrast to their old standard of 0 °C and 101.325 kPa (1 atm). Another example is from the oil industry. While a standard of 60 °F and 14.696 psi was used in the past, the current usage (particularly in North America) is predominantly of 60 °F and 14.73 psi.

Natural gas companies in Europe and South America have adopted 15 °C (59 °F) and 101.325 kPa (14.696 psi) as their standard gas volume reference conditions. Also, the International Organization for Standardization (ISO), the United States Environmental Protection Agency (EPA) and National Institute of Standards and Technology (NIST) each have more than one definition of standard reference conditions in their various standards and regulations.

The SATP used for presenting chemical thermodynamic properties (such as those published by the National Bureau of Standards) is standardized at 100 kPa (1 bar) but the temperature may vary and usually needs to be specified separately if complete information is desired (see standard state). Some standards are specified at certain humidity level.



Table 1: Standard reference conditions in current use
Temperature Absolute pressure Relative humidity Publishing or establishing entity
°C kPa % RH
0 100.000   IUPAC (present definition)
0 101.325   IUPAC (former definition), NIST, ISO 10780
15 101.325 0 ICAOmarker's ISA, ISO 13443, EEA, EGIA
20 101.325   EPA, NIST
25 101.325   EPA
25 100.000   SATP
20 100.000 0 CAGI
15 100.000   SPE
20 101.3 50 ISO 5011
°F psi % RH
60 14.696   SPE, U.S. OSHA, SCAQMD
60 14.73   EGIA, OPEC, U.S. EIA
59 14.503 78 U.S. Army Standard Metro
59 14.696 60 ISO 2314, ISO 3977-2"Gas turbines – Procurement – Part 2: Standard reference conditions and ratings", ISO 3977-2:1997 and "Gas turbines - Acceptance tests", ISO 2314:1989, Edition 2, International Organization for Standardization, Geneva, Switzerland ISO
°F in Hg % RH
70 29.92 0 AMCA, air density = 0.075 lbm/ft³. This AMCA standard applies only to air.


Notes:
  • EGIA: Electricity and Gas Inspection Act (of Canada)
  • SATP: Standard Ambient Pressure and Temperature


International Standard Atmosphere

In aeronautics and fluid dynamics the term "International Standard Atmosphere" is often used to denote the variation of the principal thermodynamic variables (pressure, temperature, density, etc.) of the atmosphere with altitude at mid latitudes.

Standard laboratory conditions

Due to the fact that many definitions of standard temperature and pressure differ in temperature significantly from standard laboratory temperatures (e.g., 0 °C vs. ~25 °C), reference is often made to "standard laboratory conditions" (a term deliberately chosen to be different from the term "standard conditions for temperature and pressure", despite its semantic near identity when interpreted literally). However, what is a "standard" laboratory temperature and pressure is inevitably culture-bound, given that different parts of the world differ in climate, altitude and the degree of use of heat/cooling in the workplace. For example, schools in New South Walesmarker, Australia use 25 °C at 100 kPa for standard laboratory conditions.

Molar volume of a gas

It is equally as important to indicate the applicable reference conditions of temperature and pressure when stating the molar volume of a gas as it is when expressing a gas volume or volumetric flow rate. Stating the molar volume of a gas without indicating the reference conditions of temperature and pressure has no meaning and it can cause confusion.

The molar gas volumes can be calculated with an accuracy that is usually sufficient by using the universal gas law for ideal gases. The usual expression is:

P V = nRT

…which can be rearranged thus:

\frac{V}{n} = \frac{RT}{P}

where (in SI metric units):
P = the absolute pressure of the gas, in Pa
n = amount of substance, in mol
V = the volume of the gas, in m3
T = the absolute temperature of the gas, in K
R = the universal gas law constant of 8.3145 m3·Pa/(mol·K)
or where (in customary USA units):
P = the absolute pressure of the gas, in psi
n = number of moles, in lbmol
V = the volume of the gas, in ft3/lbmol
T = the absolute temperature of the gas absolute, in °R
R = the universal gas law constant of 10.7316 ft3·psi/(lbmol·°R)


The molar volume of any ideal gas may be calculated at various standard reference conditions as shown below:

  • V/n = 8.3145 × 273.15 / 101.325 = 22.414 m3/kmol at 0 °C and 101.325 kPa
  • V/n = 8.3145 × 273.15 / 100.000 = 22.711 m3/kmol at 0 °C and 100 kPa
  • V/n = 8.3145 × 298.15 / 101.325 = 24.466 m3/kmol at 25 °C and 101.325 kPa
  • V/n = 8.3145 × 298.15 / 100.000 = 24.790 m3/kmol at 25 °C and 100 kPa
  • V/n = 10.7316 × 519.67 / 14.696 = 379.48 ft3/lbmol at 60 °F and 14.696 psi
  • V/n = 10.7316 × 519.67 / 14.730 = 378.61 ft3/lbmol at 60 °F and 14.73 psi


The technical literature can be confusing because many authors fail to explain whether they are using the universal gas law constant R, which applies to any ideal gas, or whether they are using the gas law constant Rs, which only applies to a specific individual gas. The relationship between the two constants is Rs = R / M, where M is the molecular weight of the gas.

The US Standard Atmosphere uses 8.31432 m3·Pa/(mol·K) as the value of R for all calculations. (See Gas constant)

References

  1. "Natural gas – Standard reference conditions", ISO 13443, International Organization for Standardization, Geneva, Switzerland   ISO Standards Catalogue
  2. "Extraction, First Treatment and Loading of Liquid & Gaseous Fossil Fuels", Emission Inventory Guidebook B521, Activities 050201 - 050303, September 1999, European Environmental Agency, Copenhagen, Denmark   Emission Inventory Guidebook
  3. "Electricity and Gas Inspection Act", SOR/86-131 (defines a set of standard conditions for Imperial units and a different set for metric units)   Canadian Laws
  4. "Standards of Performance for New Sources", 40 CFR--Protection of the Environment, Chapter I, Part 60, Section 60.2, 1990   New Source Performance Standards
  5. "Design and Uncertainty for a PVTt Gas Flow Standard", Journal of Research of the National Institute of Standards and Technology, Vol.108, Number 1, 2003   NIST Journal
  6. "National Primary and Secondary Ambient Air Quality Standards", 40 CFR--Protection of the Environment, Chapter I, Part 50, Section 50.3, 1998   National Ambient Air Standards
  7. "Table of Chemical Thermodynamic Properties", National Bureau of Standards (NBS), Journal of Physics and Chemical Reference Data, 1982, Vol. 11, Supplement 2.
  8. "Glossary", 2002, Compressed Air and Gas Institute, Cleveland, OH, USA   Glossary
  9. The SI Metric System of Units and SPE Metric Standard (Notes for Table 2.3, on PDF page 25 of 42 PDF pages, define two different sets of reference conditions, one for the standard cubic foot and one for the standard cubic meter)
  10. "Air Intake Filters", ISO 5011:2002, International Organization for Standardization, Geneva, Switzerland ISO
  11. "Storage and Handling of Liquefied Petroleum Gases" and "Storage and Handling of Anhydrous Ammonia", 29 CFR--Labor, Chapter XVII--Occupational Safety and Health Administration, Part 1910, Sect. 1910.110 and 1910.111, 1993   Storage/Handling of LPG
  12. "Rule 102, Definition of Terms (Standard Conditions)", Amended December 2004, South Coast Air Quality Management District, Los Angeles, California, USA   SCAQMD Rule 102
  13. "Annual Statistical Bulletin", 2004, Editor-in-chief: Dr. Omar Ibrahim, Organization of the Petroleum Exporting Countries, Vienna, Austria   OPEC Statistical Bulletin
  14. "Natural Gas Annual 2004", DOE/EIA-0131(04), December 2005, U.S. Department of Energy, Energy Information Administration, Washington, D.C., USA   Natural Gas Annual 2004
  15. "Effects of Altitude and Atmospheric Conditions", Exterior Ballistics Section, Sierra's "Rifle and Handgun Reloading Manual, 5th Edition", Sedalia, MO, USA   Exterior Ballistics
  16. The pressure is specified as 750 mmHg. However, the mmHg is temperature dependant, as mercury expands as temperature goes up. Here the values for the 0-20°C range are given.
  17. ANSI/AMCA Standard 210, "Laboratory Methods Of Testing Fans for Aerodynamic Performance Rating", as implied here: http://www.greenheck.com/pdf/centrifugal/Plug.pdf when accessed on October 17, 2007
  18. The standard is given as 29.92 inHg at an unspecified temperature. This most likely corresponds to a standard pressure of 101.325 kPa, converted into ~29.921 inHg at 32 °F)
  19. Fundamental Physical Properties: Molar Volumes (CODATA values for ideal gases as listed on a NIST website page)


See also



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